(1) Field of the Invention
The present invention relates to a process for producing a glass mold. More particularly, the present invention relates to a process for producing a glass mold with which a molded glass article of high precision requiring no post-operation (e.g. polishing) can be obtained by press molding.
(2) Prior Art
Molds used for producing, by press molding, a molded glass article of high precision requiring no polishing must be chemically inactive to a glass to be molded at high temperatures, and must not invite adhesion of glass to the mold after press molding, because the press molding is ordinarily effected at high temperatures such as 300-700.degree. C. though the molding temperature differs by the composition of the glass to be molded. Also, materials for a base for glass molds are required to be processable to an optically high precision surface, to withstand impact applied during press molding and to cause no thermal deformation, and thus must have sufficient heat resistance, sufficient thermal shock resistance and sufficient mechanical strength.
As the materials for glass mold, there have conventionally been used WC (tungsten carbide), a WC-Co alloy and various cermets. These materials, however, have a drawback of causing surface roughening by oxidation at high temperatures. As a material for glass mold which causes no surface roughening at high temperatures and which can be processed into a mold having a mirror surface, there can be mentioned one having thereon a .beta.-SiC film formed by a CVD method. Even this material, however, has a drawback in that the mold produced therefrom invites adhesion of glass thereto when glass is press-molded in the mold at high temperatures of 400.degree. C. or above.
As a means for preventing adhesion of glass to the mold, it is known to form a releasing film of carbon on the surface of a mold. As the releasing film of carbon, a diamond film and a diamond-like carbon film (hereinafter referred to as i-carbon film, in some cases) are disclosed in Japanese Patent Application Kokai Nos. 281030/1986 and 183134/1986. However, each of these films has the following problems.
For example, in the case of the diamond film, which is clearly shown to be polycrystalline diamond by means of Raman spectroscopy and SEM observation, the film cannot be obtained so as to have a smooth surface and, in order to produce therefrom a glass mold with an optically good surface precision, polishing is necessary. However, since the diamond film per se is very hard, the polishing is very difficult and accordingly the mold obtained is very expensive and impractical to produce. In the case of the glass mold with a diamond-like carbon film (an i-carbon film), when a glass is hot-pressed in the mold at 500-700.degree. C. (even if it is effected in a H.sub.2 gas atmosphere), the diamond-like carbon film which inherently has poor heat resistance and durability suffers from deterioration in properties with the lapse of time, and the diamond-like carbon film peels off.
As described in detail above, the conventional diamond film and diamond-like carbon film used as a releasing film for a glass mold, are not satisfactory in formation of an optical mirror surface on a mold base, heat resistance, durability, adhesivity to the mold base, productivity (economy), etc.